Use WAL mode for SQLite cache databases#13800
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With "truncate" mode, if we try to write to the database while another process has an active write transaction, we'll block until the other transaction finishes. This is a problem for the evaluation cache in particular, since it uses long-running transactions. WAL mode does not have this issue: it just returns "busy" right away, so Nix will print error (ignored): SQLite database '/home/eelco/.cache/nix/eval-cache-v5/...' is busy and stop trying to write to the evaluation cache. (This was the intended/original behaviour, see AttrDb::doSQLite().)
This avoids problems with older versions of Nix that don't put the caches in WAL mode. That's generally not a problem, until you do something like nix build --print-out-paths ... | cachix which deadlocks because cachix tries to switch the caches to truncate mode, which requires exclusive access. But the first process cannot make progress because the cachix process isn't reading from the pipe.
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I am not really the one to judge here, as I don't have much hands-on experience with the eval cache, but we use "wal" elsewhere, and the commit messages / PR description makes sense.
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Motivation
With "truncate" mode, if we try to write to the database while another process has an active write transaction, we'll block until the other transaction finishes. This is a problem for the evaluation cache in particular, since it uses long-running transactions. I.e. other Nix commands operating on the same flake could be blocked for an unbounded amount of time.
WAL mode does not have this issue: it just returns "busy" right away, so Nix will print
and stop trying to write to the evaluation cache. (This was the intended/original behaviour, see
AttrDb::doSQLite().)A quick benchmark (running
nix search github:NixOS/nixpkgs/bf8462aeba50cc753971480f613fbae0747cffc0?narHash=sha256-bPyv7hsbtuxyL6LLKtOYL6QsmPeFWP839BZQMd3RoUg%3D20 times) did not show a significant performance difference:This PR also updates the versions of the SQLite caches. This avoids problems with older versions of Nix that don't put the caches in WAL mode. That's generally not a problem, until you do something like
which deadlocks because cachix tries to switch the caches to truncate mode, which requires exclusive access. But the first process cannot make progress because the cachix process isn't reading from the pipe.
Based on DeterminateSystems#150, DeterminateSystems#167.
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